Heredity
What you must understand
Heredity explains how traits pass from parents to offspring. Know the structure of DNA, the process of DNA replication (semi-conservative), transcription and translation, and how genes determine proteins and therefore phenotype.
Understand meiosis and how it produces genetic variation through crossing over and independent assortment. Distinguish meiosis from mitosis. Know the difference between genotype and phenotype, dominant and recessive alleles, homozygous and heterozygous, and how to use Punnett squares.
Understand inheritance patterns: complete dominance, incomplete dominance, codominance, sex linkage and multiple alleles (e.g. ABO blood groups). Know how to interpret pedigrees and predict offspring ratios.
Traps in questions
- "Dominant" does not mean "more common"; it means expressed when both alleles are present.
- Meiosis produces haploid gametes; mitosis produces identical diploid cells.
- In incomplete dominance, the heterozygote shows an intermediate phenotype; in codominance, both traits are expressed.
- Sex-linked traits are carried on sex chromosomes; males are more often affected by X-linked recessive traits.
- Always distinguish genotype ratios from phenotype ratios.
Rule of thumb
For any cross, write the parental genotypes, list the gametes, then draw the Punnett square - and state ratios separately for genotype and phenotype.
Worked example
Cross two heterozygous organisms (Aa × Aa) for a completely dominant trait. Gametes are A and a. Offspring genotypes are 1 AA : 2 Aa : 1 aa, and phenotype ratio is 3 dominant : 1 recessive.